Modified the image_scroll_producer

Modified the image_scroll_producer
==================================

* The SPEED parameter is now expressed as pixels to move per frame/field.
  Previously it was expressed as number of half pixels to move per frame.
* The scrolling direction is now reversed so that a positive speed is now
  correct for an ordinary end credits roll.
* Previously both fields in interlaced video formats represented the same
  temporal image position causing non smooth film like motion. This has now
  been corrected so that both fields represents two different temporal image
  positions for smooth field-rate motion instead of frame-rate motion.
* The image now starts at a position just outside of the screen and ends at a
  position just outside of the screen. Previously it started at a strange
  somewhere in the middle of the image position for end credit rolls.
* The SPEED parameter now is a decimal value instead of an integer value.
  using a value of for example 0.5 will move the image one half pixel each
  frame/field placed with subpixel accuracy. SPEED 0.5 is the previous
  equivalent of SPEED -1 since the speed previously was measured in half
  pixels instead of pixels.
* The producer now internally is more exact in the positioning of each
  frame/field. Previously the image could be placed a little bit "off" because
  of double precision rounding errors (now calculates offsets in pixels instead
  of screens, which is more accurate) which could make the image being rendered
  in between two pixels with subpixel accuracy once in a while even though
  speed was 2 or more (no half pixels should be involved).
* Added support for motion blur via a new BLUR parameter with the number of
  pixels to blur as parameter.
* Added support for premultiplying the color channels of the input image with
  its alpha, useful when the software used to creates the image saves with
  straight alpha (caspar expects premultiplied alpha). This is done with the
  parameter PREMULTIPLY.

git-svn-id: https://casparcg.svn.sourceforge.net/svnroot/casparcg/server/trunk@3206 362d55ac-95cf-4e76-9f9a-cbaa9c17b72d